1. Field of the Invention
The present invention relates to a frequency-stabilized laser device with improved stability over variations in ambient temperature and so forth, a laser frequency stabilizing method and a laser frequency stabilizing program.
2. Description of the Related Art
A continuous-wave oscillation, 532-nm range solid laser uses a Nd:YAG crystal or the like as a gain medium, which is pumped with a semiconductor laser. The wavelength of such the solid laser is utilized as the standard for lengths. An actual measurement using a wavelength of the laser requires the laser at a single frequency, that is, in a single longitudinal mode. Further, stabilization of the laser light frequency using an atomic or molecular absorption spectrometry requires the frequency arbitrarily selectable.
Methods of selecting among oscillation modes of the laser for simplification include one that uses etalons in a cavity and one that uses gratings in a cavity as well known. In selection of an arbitrary frequency, a frequency filter including an optical element such as the etalon is used to select the frequency of the laser passed therethrough, and the length of the cavity is controlled, thereby controlling the frequency of the laser light.
The use of the laser light wavelength for length measurement requires higher frequency stability of the laser light in order to reduce uncertainty of the length measurement. In an iodine-stabilized laser device using an iodine molecular absorption spectrometry, the frequency can be controlled at the center of a saturated absorption signal to produce a laser light with high frequency stability (see, for example, JP2001-274495A and JP 2000-261092A).
The frequency of the laser light can be controlled through control of the cavity length of the laser cavity. For example, the temperature on a cabinet of the laser cavity is controlled to suppress variations in size of the cabinet. In addition, the displacement of a mirror-attached actuator is controlled such that the frequency meets the center of the saturated absorption signal, thereby controlling the cavity length.
Though, variations in size of the cabinet due to fluctuations of the ambient air temperature and drifts of electric signals in an actuator driver circuit and an actuator controller may cause a variation in the voltage applied to the actuator. If this variation falls within a range of voltages applied to the actuator, the cavity length can be controlled. If the voltage applied to the actuator is saturated, however, the cavity length can not be controlled and the frequency can not be stabilized at the center of the saturated absorption signal.
On the other hand, a method may be considered for increasing the range of voltages applied to the actuator, thereby avoiding the saturation of the voltage applied to the actuator and increasing the maximum displacement of the actuator. In this method, however, the increase in the applied voltage range requires improvements in S/N ratios in the actuator driver circuit and the power source to make the noise component in the displacement equal to that before the control.
Another method may also be considered for using an actuator with a larger amount of displacement per applied voltage, thereby increasing the maximum displacement of the actuator. In this method, however, if the S/N ratio in the actuator driver circuit is equal to that before the control, the noise component in the displacement becomes relatively larger. Accordingly, the S/N ratio in the driver circuit should be improved to make the noise component in the displacement equal to that before the control. In addition, the drifts of electric signals should be reduced as well.